This invention relates to colored hot melt ink images and, more particularly, to processing of transparent colored hot melt ink images after they have been printed to provide improved image quality.
Hot melt inks, also known as phase-change inks, are used in thermal transfer printers and in certain ink jet printers. The characteristic of these inks is that they are solid at room temperature, liquefied by heating for marking, and resolidified by freezing on the marked substrate.
A transparent colored hot melt ink image, such as a subtractive color image composed of cyan, magenta and yellow inks, created on the surface of a substrate may be composed of individual drops of the ink as supplied in the ink jet drop-generating process, couples of drops, lines of drops or large areas covered completely by drops. Even when the entire surface of a portion of the substrate is covered with ink, the surface of the ink tends to retain the curvature of the individual drops. Light passing through the surface of the deposited ink is refracted by the local curvature of the ink surface.
A first deficiency of transparent colored ink images having such curved surfaces occurs in the projection of color transparencies because the light is deflected by the curved surface portions from its original direction and may be lost from the optical path of the projection apparatus. The projected image of this area of the transparency appears dark. If the layer of ink has a substantially uniform thickness, light passes through the substrate and the ink in a rectilinear manner and no light is lost by refraction. Consequently, all of the light is collected by the projection lens. Hence it is advantageous if the curvatures of contiguous drops forming the region of the ink image are eliminated over the entire surface of the solid areas of the image. For individual drops of specified volume, a large radius of curvature corresponds to a small contact angle between the ink surface and the transparency substrate, as described in the Fulton et al. U.S. Pat. No. 4,873,134, the disclosure of which is incorporated by reference herein.
In the case of black-and-white transparencies, the major concern is that the deposited ink be able to block or reduce transmission of light through the transparency. However, for the projection of colored images, it is necessary for the ink to absorb selected wavelengths and pass significant fractions of the remaining wavelengths in order to produce an image with the correct colors.
When projected from a transparency, the deposited hot melt transparent colored image tends to project gray or black images because of any of three loss mechanisms, i.e., refractive scattering of transmitted light by the curved surface portions, surface losses resulting from microroughness (frosting) on the order of one micron, and bulk losses resulting from the formation of crystals within the droplet which have a different index of refraction than the other material in the droplet. The curved surface portions resulting from the three-dimensional ink spots refract light which passes through them away from the path to the projection lens so that they cast gray shadows in projection irrespective of the color of the ink which forms the image. In addition, an ink region containing an irregular surface is subject to poor adhesion, abrasion and chipping, as described in application Ser. No. 07/202,488 and in U.S. Pat. No. 4,751,528.
Flattening of the top surface portions of each of the individual ink drops on a transparent substrate in an attempt to overcome these problems is described in the published European Patent Application No. 0 308 117 and the corresponding U.S. Pat. No. 4,853,706. As described therein, each of the ink drops is subjected to pressure and/or heat in such manner that the top surface of the ink drop is flattened to provide a substantially planar region over at least 20%, and preferably 50% or 75% of the area of the support covered by the drop.
As shown in these publications, the remaining 25% to 80% of the area of each drop consists of curved surfaces and the thickness of each drop varies by up to 25%. These curved surfaces of the drop, in fact, have a greater curvature than the curvature of the original ink drop. As a result, in regions of ink patterns containing adjacent contiguous ink drops intended for 100% solid-area coverage, a substantial proportion of the light incident on the region is deflected by the remaining curved surface portions of the ink drops.
Moreover, the resulting irregular surface contributes to the tendency of the ink to crack, peel, abrade, flake and chip from the surface of the substrate. In one embodiment described in these publications, this tendency is counteracted by laminating a transparent adhesive layer over the ink drops before they are flattened, but this does not alleviate the light deflection problem resulting from the relatively large proportion of curved surface areas in the regions intended for 100% solid-area coverage.
For naturally amorphous (noncrystalline) materials, the microroughness (frosting) and bulk losses are small, i.e., the spots are glassy and "clear". Unfortunately, as is known in the art, the organic materials which are amorphous and which may be fluid enough to jet at temperatures of 100.degree. C. to 160.degree. C. tend to be very soft at room temperature. Consequently, the durability of the ink on a transparency may be inadequate. Generally, inks which have adequate hardness and which are jettable at temperatures of 100.degree. C. to 160.degree. C. are usually crystalline to a significant extent. Such high crystallinity produces light transmission losses and causes "opacity" of the ink drop. The bulk losses and surface roughness, i.e., frosting, are also a result of the ordered arrangement of the molecules into a plurality of randomly or obliquely oriented or disoriented crystals. Hence crystalline inks tend to have a high degree of surface and bulk scattering, producing light transmission losses greater than 50%, so as to project "gray" spots rather than spots with high color purity. On the other hand, such inks are generally suitable for black-and-white transparencies.
When transparent subtractive color hot melt ink images are printed on opaque substrates such as paper to provide colored reflection prints, they are subject to the same deficiencies since the light passes through the curved surface portions and any losses resulting from diffraction or scattering tend to degrade the image.